Zeolites and zeolite-like materials are microporous materials with well-defined pores up to 13 Å in diameter. Many molecules, whether in the gas or liquid phase, both inorganic and organic, have dimensions that fall within this range. Selecting a molecular sieve with a suitable pore size therefore allows separation of a molecule from a mixture based on the size of the molecule, which explains why zeolites and zeolite-like materials also are denoted “molecular sieves”. In addition to this separation mechanism, where only molecules smaller than the pores of the molecular sieve can permeate, the pores within the material can separate molecular components having different adsorption and/or diffusion properties. Apart from the selective separation of uncharged species, the well-defined pore system of the molecular sieve enables selective ion exchange of charged species and selective catalysis. In the latter two cases, significant properties in addition to the micropore diameter are important, for instance, ion exchange capacity, specific surface area and acidity. Molecular sieves can be classified in various categories, for example according to their chemical composition and their structural properties. A group of molecular sieves of particular interest is the group comprising zeolites and zeolite-like materials.
Zeolites and zeolite-like materials do not comprise an easily definable family of crystalline solids. However, the Structure Commission of the International Zeolite Association has presently approved more than 200 different zeolite framework types and assigned a 3-letter code to each framework. A criterion for distinguishing zeolites and zeolite-like materials from denser tectosilicates is based on the framework density, the number of tetrahedrally coordinated framework atoms per 1000 Å3. The tetrahedrally coordinated framework atoms are also denoted T-atoms. The maximum framework density for zeolites and zeolite-like materials ranges from 19 to over 21 tetrahedrally coordinated framework atoms per 1000 Å3, depending on the type of smallest ring present, whereas the minimum for denser structures ranges from 20 to 22. The Structure Commission maintains a zeolite structure database accessible via the internet [http://www.iza-structure.org/] and is also regularly revising and publishing the Atlas of Zeolite Framework Types. The 6th revised edition of the Atlas was published in 2007 [Ch. Baerlocher, L. B. Mc Cusker, D. H. Olson. Atlas of Zeolite Framework Types, 6th Ed., 2007, Elsevier, ISBN 978-0-444-53064-6]. Zeolite frameworks are built from TO4 tetrahedra and the T-atoms are usually silicon and aluminium atoms, but zeolite frameworks can also be prepared from only SiO4 tetrahedra. In the aluminophosphates (AlPO4), the T atoms are aluminium and phosphorous atoms. However, there are many more possibilities and atoms such as Si, Al, P, Ga, Ge, B, Be, Ti, Fe etc. can serve as T-atoms in zeolite frameworks.
Zeolites and zeolite-like materials are microporous solids with a very regular pore structure of molecular dimensions. The dimensions of the channels control the maximum size of the molecular or ionic species that can enter the pores of a zeolite. The aperture of the channels are conventionally defined by the ring size, where, for example, the term “8-ring” refers to a closed loop that is built from 8 T-atoms and 8 oxygen atoms.
Zeolites and zeolite like materials have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. These positive ions can readily be exchanged, which explains why zeolites can serve as ion exchangers.
Natural zeolite minerals are usually formed where volcanic rocks and ash layers react with alkaline groundwater. Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals, metals, quartz, or other zeolites. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential. Some of the more common zeolites found as minerals in nature are (3-letter codes within brackets) analcime (ANA), chabazite (CHA), clinoptilolite (HEU), heulandite (HEU), natrolite (NAT), phillipsite (PHI), and stilbite (STI).
Zeolites and zeolite like materials can also be prepared synthetically. A frequently prepared zeolite framework is the MFI framework, which has 10 T-atoms in the ring and thereby a suitable pore size for many applications. This framework can be prepared in pure silica form, i.e. the T-atoms are only silicon atoms. In this case, the structure is denoted silicalite-1. However, if some of the silicon atoms are replaced with aluminium atoms, the structure is denoted ZSM-5. Templates or structure directing agents are added to the reaction mixture in the synthesis of zeolites and zeolite like materials to direct the crystallization to the desired framework. For example tetrapropyl ammonium hydroxide is often used as a template in the synthesis of MFI zeolite.
There are numerous methods described in the art for the manufacture of substrate materials for inorganic catalysts and thin membranes, although there is still a need for improved substrate preparation, which is further explained herein. Substrates coated with thin films find their applications in for example the fields of membrane separation, sensor technology, catalysis, electrochemistry, ion exchange.
When the film is prepared on a porous substrate using hydrothermal synthesis techniques, a number of problems may arise. Firstly, species from synthesis solution used for growth of zeolites and zeolite-like materials may invade and be deposited within the porous substrate, resulting in reduction of the porosity of the substrate implying reduced permeation of gas or liquid molecules through the complete structure. Secondly, the substrate material may not be inert under the synthesis conditions used resulting in dissolution of atoms from the substrate to an unacceptable degree, atoms that may interfere with the intended structure of the zeolite. Thirdly, in cases where calcination is preferred to remove a structure-directing agent from the synthesized film, the invaded and deposited species in the substrate may lead to crack formation in the film and the substrate due to differences in thermal expansion properties between the species and the substrate.
Attempts to solve the first two problems have been described in the literature. WO94/25151 describes the use of a barrier layer which prevents water in the aqueous coating suspension used from entering the pores of the support to an extent such that the silica and zeolite particles form a thick gel layer on the support. The barrier layer may be temporary or permanent; temporary barrier layers were fluids such as water or glycol. By this method it was possible to synthesize thick films on the support.
Gavalas et al. describe the use of a specific barrier in zeolite membrane synthesis; “Use of diffusion barriers in the preparation of supported zeolite ZSM-5 membranes”, Journal of Membrane Science, 126 (1997), 53-65. The authors describe a method implying that a mixture of furfuryl alcohol and tetraethylorthosilicate was impregnating a support. After impregnation, the mixture was polymerized by exposure to p-toluene sulfonic acid at elevated temperature and the resulting polymer was subsequently carbonized. Before the synthesis of the zeolite membrane, the carbonized polymer was removed from the top region of the support where the zeolite membrane was to be deposited. After synthesis of the zeolite membrane, all the carbon was removed by calcination. However, TEOS became converted to a silicate and deposited within the pores of the support not covered by carbon.
Thin continuous films of zeolite or zeolite-like crystals may be produced with techniques well known in the art. However, the preparation of the substrate upon which a continuous zeolite or zeolite-like film will be synthesized may vary. It is not always necessary to mask a porous substrate upon which a film of zeolite or zeolite-like crystals is grown. However, if masking of the substrate is preferred in order to prohibit the invasion of species from the synthesis solution into the substrate used as a support for the film, the masking procedure may involve filling the pores in the substrate with polymers or wax. Such a procedure is described in WO 00/53298. This rather time consuming and practically complicated procedure implies that the top layer of the substrate, where the zeolite film is to be synthesized, is first covered with a thin layer of a polymer that is soluble in polar aprotic solvents, such as acetone. The polymer may be polymethylmetacrylate (PMMA) and the solvent might be acetone, as described in WO 00/53298. Subsequently, the porous substrate may be filled with paraffin wax, which is not soluble in solvents such as acetone. To be able to deposit seed crystals necessary for film growth onto the substrate surface, the first layer (PMMA) was removed since the method described in WO 00/53298 relied on the attachment of a cationic polymer onto the bare substrate surface. The cationic polymer facilitates the deposition of seed crystals onto the substrate surface, because of the well-known electrostatic (Coulomb) attraction between oppositely charged sites. From these seed crystals, a membrane film could grow when the substrate with seed crystals was immersed into a suitable synthesis solution under hydrothermal conditions. However, a drawback with this masking procedure is not only the time required to mask a substrate, but also the depth precision and masking efficiency and in addition, the method is difficult to apply on supports with complex geometries, such as multi-channel tubes needed applications. Ideally the PMMA layer, brought onto the top layer of the substrate from acetone solution, should coat the top layer of the porous substrate surface without invading (entering) the small pores of the top layer (e.g. 100 nm pores) of the support and the wax should ideally impregnate the porous substrate completely up to the PMMA layer. Experience shows that this ideal masking sometimes is very hard to accomplish in practice. PMMA dissolved in acetone tends to invade (penetrate into) the top layer of for example a porous α-alumina substrate. In practice, the penetration of PMMA into the top layer may vary from essentially zero to several micrometers into the support. This results in incomplete filling of the support with wax and subsequent invasion of species from the synthesis solution and/or film growth into the substrate pores during film growth. A problem here is that a thicker zeolite membrane will imply a lower flux and permeance through the membrane than was desirable. A lower permeance implies that for example less of CO2 can be extracted from a feed of synthesis gas within a given time period, which affect the cost efficiency of the membrane process.
Another problem with the masking concept described above is the lack of control of the thickness of the synthesized membrane, since the invasion of species from synthesis solution may vary between different synthesized films/membranes as well as between different parts of the same film/membrane.
A further problem associated with the masking concept described above and the partial invasion of the synthesis solution in the support is the fact that the conditions required for synthesizing numerous zeolite framework films are so severe that the substrate is partly dissolved or etched. Accordingly, aluminium may be dissolved from the top layer of a non-masked or incompletely masked α-alumina substrate leading to a zeolite structure that was not intended to be produced.
A further problem with the masking concept described above is that it is difficult to apply to supports with complex geometries, such as multichannel tubes, needed to create large membrane surfaces for practical application of the membranes. Examples of commercially available multichannel alumina tubes are those sold by Inopor GmbH. Such tubes may for instance comprise 19 channels with a diameter of only 3.5 mm and a length of 1.2 m. It is straightforward to realize that it will be difficult to coat these narrow and long channels with PMMA perfectly as required for membrane preparation. Incomplete coating with PMMA would lead to protrusion of wax inside the channels, which would result in incomplete coverage of the channels with film and thereby defective membranes.
WO97/33684 discloses a procedure for preparing molecular sieve films, wherein microcrystals are attached on a substrate having a surface charge opposite to the charge of the microcrystals. Substrates with surface charge are hydrophilic. Subsequently, a molecular sieve film is allowed to grow on the substrate.
Accordingly, there is still a need in the art to produce films and membranes which avoid or minimizes the problems mentioned above, which are associated with known procedures for preparing films comprising zeolite and/or zeolite-like crystals on porous substrates. There is also a general need in the art to provide alternative measures for pretreatment of substrates or supports onto which subsequently seed crystals are to be deposited.